1. Field of the Invention
The present invention relates to substrates having a photocatalytic, TiO2-containing layer, which have improved photocatalytic activity, and to processes for their production.
2. Discussion of Background Information
The photocatalytic properties of TiO2 particles have long been known in the literature and have been intensively investigated. The photocatalytic effect is based on a semiconductor property of TiO2, by which a photon forms a hole-electron pair which has a relatively long recombination time. Diffusion of holes and electrons to the surface sets in motion processes which develop strongly oxidative action directly or indirectly via water with subsequent hydrogen peroxide formation. The oxidation potential at over 3 eV is so high that virtually all organic substances which come into contact with such TiO2 particles are oxidized. However, this process only proceeds when a notable UV light fraction is present in the incident light. Since the fraction of UV light in visible light is relatively small, the photocatalytic action as a result of the incident photons is limited. The recombination of the electrons with the holes further reduces the efficiency.
It has also been found that it is difficult, on substrates or surface layers which are themselves oxidizable, for example in the case of substrates or layers of organic polymers, to prevent oxidation by a photocatalytic layer applied thereto and thus damage to the substrate or to the layer. Even in the case of substrates or surface layers of glass, direct application of the photocatalytic layer has the disadvantage that sodium ions in the glass can diffuse into the photocatalytic layer, which damages the glass and/or disrupts the photocatalytic process.
It is therefore desirable to achieve an increased photocatalytic activity and/or to provide protection for substrates or surface layers which are sensitive toward the photocatalytic layer.